As a method of detecting defects of a circuit pattern formed on a wafer due to an image comparison inspection in a process of manufacturing a semiconductor device, electron beams are illuminated onto the specimen with the results that there remains fine etching residuals which is equal to or smaller than the resolution of an optical microscope, thereby making it possible to detect a configuration defect such as a fine pattern defect or detect an electric defect such as a non-aperture defect of a fine through-hole.
In this example, in a system using a scanning electron microscope that scans the specimen with point electron beams, there is a limit to obtain a practical inspection speed. Therefore, there has been proposed a device that conducts inspection at a high-speed through a so-called projection system such that a rectangular electron beam is illuminated onto a semiconductor wafer, and secondary electrons, backscattered electrons or electrons that are reflected without being illuminated onto the wafer due to the production of a reverse electric field are imaged by a lens (refer to, for example, Japanese Patent Application Laid-Open No. 7-249393, Japanese Patent Application Laid-Open No. 10-197462, and Japanese Patent Application Laid-Open No. 2003-202217).
However, the projection system using the secondary electrons or the mirror electrons suffers from the following problems.
The device that magnifies and projects the image of the secondary electrons or backscattered electrons as the detected electrons is called “low energy electron microscope”. In this system, it is expected that images are formed rapidly compared with that in an SEM (scanning electron microscope) because a beam current is larger than that of the SEM, and the images can be acquired at once. However, the emission angle distribution of the secondary electrons are spread at a wide angle, and the energy is also wide spread from about 1 to 10 eV. It can be readily understood that the image with sufficient resolution cannot be obtained unless most of secondary electrons are cut (reference document: FIG. 6 disclosed in “LSI testing symposium/1999 conference minutes, P142”). The resolution is substantially 0.2 μm when a voltage supplied to a specimen is −5 kV according to a relationship between the imaging resolution of the secondary electrons and a negative voltage supplied to the specimen for accelerating the secondary electrons that have been emitted from the specimen.
Then, all of the secondary electrons cannot be used for image formation. For example, in the calculation of the above document, a beam having an opening angle of 1.1 mrad or less is used in an image plane that has passed through an objective lens. The secondary electrons within a range of the opening angle are about 10% of the entire secondary electrons at the highest. In addition, calculation is made assuming that the energy width of the secondary electrons used for imaging is 1 eV, but the energy width of the secondary electrons actually has several eV or more for emission, and a base at the higher energy side exists up to 50 eV. In the case where only the secondary electrons having the energy width of 1 eV at the highest are extracted among the secondary electrons having the above wide energy distribution, the secondary electrons is further reduced to several tens percentages.
As described above, even if the images are going to be formed at once by using the secondary electrons that are obtained by illuminating a large current to the specimen with the electron beams as an area beam, it is difficult to ensure the S/N ratio of the image because the ratio of electrons that can actually contribute to the image formation is low, as a result of which it is impossible to reduce the inspection time as much as can be expected. Even if the backscattered electrons are used for the image formation, only the amount of the backscattered electrons which is smaller than the illuminating beam current by two digits is obtained, and it is difficult to perform both of the high resolution and the high throughput as in the case of the secondary electrons.
The device that magnifies and projects the image of mirror electrons reflected immediately before the specimen without abutting against the specimen instead of the secondary electrons or the backscattered electrons is called “mirror electron microscope”. The defect can be detected by detecting the disturbance of a potential or a configuration which is caused by the defect by means of the mirror electrons. In the case where the pattern is convexed or negatively charged, an equipotential surface formed immediately above the specimen acts as a convex lens with respect to the incident electrons. In the case where the pattern is concaved or relatively positively charged, the equipotential surface formed immediately above the specimen acts as a concave lens with respect to the incident electrons. As described above, the mirror electrons slightly change the trajectory due to the lens formed immediately above the specimen, but when the focal point conditions of the imaging lens are adjusted, most of those mirror electrons can be used for image formation. That is, the use of the mirror electrons makes it possible to obtain an image that is high in the S/N ratio and expect the reduction of the inspection time.
However, the image that is obtained from the mirror electrons reflects the equipotential surface immediately above the specimen and is greatly different from a general electron microscope. As a result, it is difficult to obtain information in correspondence with an accurate configuration and position of the specimen. Accordingly, it is essential to provide means for acquiring the accurate configuration and the positional information of the specimen in addition to a function of acquiring the image of the mirror electrons.
The device structure of the mirror electron microscope that magnifies and projects the mirror electrons for imaging is made up of an illuminating lens system that illuminates the electron beams onto the specimen, an imaging lens system that images the electrons reflected from the specimen, and a separator that separates the illuminating electron beams from the reflected electron beams. The same device structure is disclosed in the above-mentioned conventional examples of the low energy electron microscope that images the secondary electrons or the backscattered electrons (the above three Japanese Patent publications) is composed of the illuminating lens system, the imaging lens system, and the separator of an E×B deflector that forms the orthogonal electric field to the magnetic field.
However, the above-mentioned conventional examples have no device that can perform both of the mirror electron microscope and the low energy electron microscope, and the mirror electron and the secondary electron image cannot be observed in the same visual field. In the mirror electron microscope, an accelerating voltage V0 that is applied to an electron source is set to be substantially the same potential as a specimen supply voltage Vs with the results that the illuminating electron beam is reversed immediately above the specimen into a reflecting electron beam, and the reflecting electron beam is exitted from an objective lens with the same energy eV0 as an energy eV0 that is entered to the objective lens. On the other hand, in the low energy electron microscope, the voltage V0 that is applied to the electron source is set to a negative voltage with respect to the specimen supply voltage Vs with the results that the electron beam is illuminated to the specimen with an energy of e(Vs−V0). When it is assumed that the energy of the secondary electrons or the backscattered electrons which are exitted from the specimen is eV2, the secondary electrons or the backscattered electrons enter a separator after having exitted from the objective lens with the energy of (Vs+V2). Accordingly, even if the specimen supply voltage Vs is adjusted to meet the low energy electron microscope conditions from a mode for acquiring the specimen image by means of the mirror electron microscope, the energy of the reflecting electron beam that passes through the separator is changed. As a result, a deflection action occurs in the reflecting electron beam, and the magnified image is moved, thereby making it difficult to observe the secondary electron image.
The present invention has been made in view of the above problems, and therefore an object of the present invention is to provide an electron microscope that can perform both of a mirror electron microscope observation and a low energy electron microscope observation in the same visual field. Another object of the present invention is to provide an electron beam inspection system that can detect a defective portion of a pattern formed on a specimen with a high resolution and at a high speed by means of the electron microscope.